Air percussion drilling assembly

ABSTRACT

An air percussion hammer drill is disclosed for operation in an earthen formation. The air compression hammer mechanism comprises a piston that reciprocates while simultaneously rotating within its housing. A hammer drill bit slidably keyed to the bottom of the piston transfers the impact energy to the formation and rotates during operation independent of an attached drill string.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an air compression hammer drill tool. Moreparticularly, this invention relates to a downhole air compressionhammer that impacts while simultaneously rotates the bit, therebyassuring maximum penetration of the bit in an earthen formationindependent of the rotation of the drill string.

2. Description of the Prior Art

In percussion drilling the rock cutting mechanism is of an impactingnature rather than shearing. Therefore, the drill bit rotationalparameters, e.g. torque and rpm, are not relevant from a rock formationbreaking point of view, except for the necessity that the cuttingelements of the bit need to be "indexed" to fresh rock formations. Instraight hole air drilling, and especially in mining, this need isachieved by rotating the drill string slowly. This is accomplished inconventional hammer bit operations by incorporated longitudinal splineswhich key the bit body to a cylindrical sleeve at the bottom of thehammer (commonly known as the sub driver). The drill string rotation isthen transferred to the hammer bit itself. Experience has proven thatthe bit optimum rotational speed is approximately 20 rpm for an impactfrequency of 1600 bpm (beats per minute). This rotational speedtranslates to an angular displacement of approximately 4 to 5 degreesper impact of the bit against the rock formation. Another way to expressthis rotation is the cutters positioned on the outer row of the hammerbit move at the rate of one half the cutter diameter per stroke of thehammer.

An example of a typical hammer bit connected to a rotatable drill stringis described in U.S. Pat. No. 4,932,483. The downhole hammer comprises atop sub and a drill bit separated by a tubular housing incorporating apiston chamber therebetween. A feed tube is mounted to the top sub andextends housing and over the feed tube. Fluid porting is provided in thefeed tube and the piston to sequentially admit fluid in a first spacebetween the piston and top sub to drive the piston towards the drill bitsupport and to a second space between the piston and the drill bitsupport to drive the piston towards the top sub.

Rotary motion is provided to the hammer assembly and drill bit by theattached drill string powered by a rotary table typically mounted on therig platform.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a hammer assembly thatreciprocates and simultaneously rotates a hammer bit.

It is another object of this invention to provide an air percussiondrilling assembly wherein the hammer mechanism converts axial motion ofthe reciprocating piston to rotary motion of an extended portion of thebit body as the bit works in a borehole.

The kinetic energy of the reciprocating piston is employed to rotate thebit. The linear motion of the piston is converted into rotational motionby using one or more helical grooves formed by the piston body. Toprevent the piston from oscillating in the rotary mode, an indexingclutch mechanism is provided to induce bit rotation in one directiononly.

The upper portion of the hammer bit (normally splined) is replaced by ashaft that is slidably engaged with and keyed to a complimentarilyshaped female receptacle or bore formed by the lower portion of apiston. The shaft of the hammer is therefore, slidably engaged at alltimes, to the base of the piston and is so designed to be rotated by theindexing piston with a minimum of drag. Thus, axial motion between thepiston and bit body is allowed but relative rotational motion is not,i.e. the bit would rotate if the piston rotate and vice versa. One ormore longitudinal helical grooves are machined on the piston uppersection. These grooves are keyed to an inner race of a "sprag" clutchassembly via dowel pins or spherical balls. The outer race of thisclutch assembly is locked to the inner bore of a cylindrical hammerhousing. The clutch sprags are set to clockwise motion and to preventcounter clockwise rotational movement of the inner race with respect tothe outer race.

The downward motion of the piston, (the piston being coupled to theclutch through interaction between the helical groove, the engaged balland the clutch) mandates either a counter clockwise rotation of theinner race or a clockwise rotation of the piston. Since counterclockwise rotation of the inner race is not possible, the piston mustrotate clockwise when the piston moves downward. Similarly, the upwardmotion of the piston requires either the clockwise rotation of the innerrace or the counter clockwise rotation of the piston. Since the frictionagainst the clockwise rotation of the inner race is significantly lessthan that against the piston/bit rotation, the inner race would rotateclockwise and allows the piston to move straight upward. Therefore, onthe downstroke of the piston the bit is forced to rotate clockwise;while on the upstroke the inner race rotates instead, thereby preventingthe bit from "turning back".

An air percussion hammer apparatus with means for rotating the hammerbit while its piston reciprocates in a housing independent of anattached drill string is disclosed. The bit rotating means consists of acylindrical housing forming a first open up-stream end connectable to adrill string component and a second downstream end, the second endcontaining the hammer bit.

A pneumatic feed tube forms a first open end and a second substantiallyclosed end, the first end of the feed tube being concentric with andfixed within the housing. The feed tube is positioned toward the firstup-stream end of the housing, the second end of the feed tube forms oneor more metered openings between the first and second ends of the feedtube.

A piston body is slidably retained within a first sleeve formed by thehousing. The piston body forms first and second open ends with the firstend being concentrically retained and slidably engaged with the secondend of the feed tube. The second downstream end of the piston forms ahammer striking surface. The piston further forms at least one axiallyoriented helical groove in an outside wall of the first upstream end ofthe piston and a pair of pneumatic communication ports between anoutside wall of the body and an interior chamber formed by the body.More specifically, one of the ports leads from an interior chamberformed by the piston toward the second end of the piston to a chamberformed between the first open end of the piston and the cylindricalhousing. The other of the ports leads from an interior chamber formedbetween an exterior wall of the piston and the sleeve formed by thehousing toward the second open end of the piston. One or the other ofthe ports in the body sequentially registers with the metered openingsin the feed tube when the reciprocating piston is moved into alignmenttherewith during an operating cycle of the apparatus. The second end ofthe piston body further forms a longitudinal sleeve therein.

A hammer bit body is slidably contained within a cylindrical sleeveformed by the second end of the cylindrical housing. The bit body formsa first upstream shaft end adapted to slidably engage the sleeve or boreformed in the bottom portion of the piston. Means are formed between theshaft of the bit and the bore of the piston to slideably key the shaftto the piston so that the bit rotates with the piston. The hammer bitbody further forms a second, bit cutter end.

A clutch means is contained within the housing and is positionedadjacent to and interconnected with the helical groove formed in thefirst end of the piston. The clutch mean serves to rotate the piston andthe bit keyed thereto, incrementally and in one direction only, eachtime the piston reciprocates within the cylindrical housing duringoperation of the air percussion apparatus.

An advantage then of the present invention over the prior art hammertools is the ability to rotate the bit independent of any rotation ofthe drillstring.

The above noted objects and advantages of the present invention will bemore fully understood upon a study of the following description inconjunction with the detailed drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the hammer mechanism and bitconnected to a drill string;

FIG. 2 is a perspective view of the hammer drive piston illustrating thehelix grooves formed in the top section of the piston and the variouspneumatic ports formed therein;

FIG. 3 is a cross-sectional view of the hammer mechanism with the bitcutter end contacting the formation, the piston being at the top of itsstroke;

FIG. 4 is a section taken through 4--4 of FIG. 1 illustrating the innerand outer air passages formed by the hammer bit body;

FIG. 5 is a section taken through 5--5 of FIG. 3 showing therelationship formed between the hammer housing and the shank of thehammer bit;

FIG. 6 is a section taken through 6--6 of FIG. 3 illustrating the clutchmechanism including the helical groove and ball engaging system thatresults in bit rotary motion converted from piston reciprocating motion;

FIG. 7 is a section taken through 7--7 of FIG. 3 illustrating the spragshoused within the clutch that prevent the piston from oscillating; theclutch mechanism insures that the piston always rotates in a clockwisedirection.

FIG. 8 is a cross-sectional view of the percussion mechanism at thetermination of one complete cycle;

FIG. 9 is a partially cutaway view of an alternative embodiment of thehammer rotary drive means, and

FIG. 10 is a view taken through 10--10 of FIG. 9 illustrating thesliding ball track mechanism between the piston and the hammer bit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS AND BEST MODE FOR CARRYING OUTTHE INVENTION

FIG. 1 illustrates an air percussion drilling assembly generallydesignated as 10. The air percussion apparatus consists of a cylindricalhousing 12 that forms an up-stream threaded female end 14 adapted to beconnected to, for example, a drill string 15. A hammer bit generallydesignated as 18 is slideably retained within the opposite or downstreamend 16 of cylindrical housing 12.

A check valve 20 is retained within housing 12 adjacent threaded end 14.Valve body 21 is biased closed by valve spring 22 when the percussionapparatus is not functioning or the apparatus is "tripped" out of theborehole to prevent water or formation detritus from backing up thedrill string.

A pneumatic feed tube generally designated as 24, is mounted within afeed tube support member 25; the support member being secured withinhousing 12. An interior chamber 28 communicates with the drill string 15at an upstream end of the housing 12 and with slotted, axially alignedopenings 26 formed in the feed tube wall at an opposite end of the tube24. A choke 27 substantially closes off the downstream end of the tubejust below the slotted openings 26.

A pneumatic piston generally designated as 30 slideably engages cylinderwall 13 formed by housing 12. The body 31 of the piston 30 forms anupper, reduced diameter cylindrical segment 32. An inner cylindricalwall 33 overlaps and partially engages the outside wall 29 of theconcentric feed tube 24. An annular chamber 35 formed by segment 32provides a pneumatic conduit for pressurized air to the slots 26 formedin feed tube 24 depending upon the axial position of the piston 30within housing 12. The piston body 31 further forms ports or conduits 38and 39 that communicate with slots 26 in tube 24. The ports directpressurized air either to slots 40 formed in the piston 30 and fromthere to chamber 41 formed below piston 30 in housing 12 or to annularchamber 37 above the piston depending on the axial position of thepiston as the mechanism cycles through its operating modes (see FIGS. 1and 2).

FIG. 1 illustrates the hammer bit 18 positioned above a borehole bottom8; the bit being suspended from retaining ring 49 attached to wall 13near the bottom of housing 12. As long as the bit remains off bottom 8,pressurized air 11 is directed down drillstring 15 into chamber 28formed in feed tube 24. The air is then directed through slots 26 toannular chamber 35 and from there to chamber 37. Ports 39 in piston 30then direct the pressurized fluid to air passage 53 formed through thecenter of hammer bit 18 then out through one or more nozzles 54 formedin the bit cutting face. The air under pressure serves to clean the rockchip debris and other detritus such as accumulated water from theborehole bottom 8 prior to commencement of further drilling operations.

As the air percussion assembly 10 is lowered down the borehole 6 formedin earthen formation 4, the bit 18 contacts bottom 8 (FIG. 3). The bit18 and piston 30 is subsequently pushed back into housing 12 a distancewherein shoulder 51 formed by the bit 18 contacts rim 16 formed byhousing 12. Upon contact, air is shut off to chambers 35 and 37 when thepiston moves over the fixed feed tube 24. The pressurized air is thenredirected down through ports 38 to slotted channels 40 into chamber 41below piston 30. The piston is then forcibly accelerated up cylinderwalls 13 separating the impact surface 34 formed at the bottom of thepiston from the top of the hammer bit 18 as illustrated in FIG. 1. Themomentum of the piston mass carries the piston 30 to the upper end ofchamber 37. Pressurized air is then redirected to the top of the piston(chamber 37) through slots 26 in feed tube 24 into piston ports 39. Thepiston then is accelerated down cylinder walls 13; end 34 of the pistonsubsequently impacting end 55 of the hammer bit 18 thereby completingthe cycle (FIG. 8).

FIG. 3 depicts the piston 30 at the top of its travel within cylindricalsleeve 13 prior to being accelerated toward impact surface 55 of hammerbit 18. As the piston moves downward toward the hammer bit, the clutchmechanism generally designated as 56, engages ball 58 with helicalgroove 36 formed in the upper reduced diameter section 32 of piston 30.The piston moves in a clockwise direction as it moves down toward thehammer bit and, since the hammer bit is keyed to the piston, the bitmoves rotationally in concert with the piston. When the piston is cycledin the reverse or upward direction, the clutch slips hence preventingthe piston (and hammer bit) from rotating in a counter clockwisedirection. The piston and hammer bit therefore is rotationally indexedin a clockwise direction only.

The piston and hammer is preferably rotated on the downstroke of thepiston for the following reasons; there is tremendous formationresistance imparted to the piston hammer mechanism on the upward cycleof the piston due to the fact that the lower chamber 41 is chargedforcing impact surfaces 34 and 55 apart subsequently driving the currentface 19 of the hammer bit into the formation thereby resisting theturning or rotational force exerted on the piston by the ball 58 inhelical groove 36. Therefore, if the rotational forces were exerted onthe piston and the bit on the downstroke, the bit is released from theformation and the rotational forces easily rotate or index the bit toits new position without unnecessary wear on the various slidingsurfaces.

FIG. 4 illustrates a section taken through housing 12 (FIG. 1) showingthe piston 30 with the shaft 50 of the hammer 18 slideably retainedwithin sleeve 42 formed by the piston. The rectangular shaped shaft 50with rounded ends, for example, is slideably retained withincomplimentarily shaped sleeve 42 formed in piston 30. The central airpassage 53 communicates with the nozzles 54 formed in the cutter face 19of hammer 18.

It would be obvious to utilize conventional hammer bit splines as ameans to key the shank of the hammer bit to the piston without departingfrom the scope of this invention (not shown).

FIG. 5 depicts a section through the hammer body 47 slideably retainedin cylindrical sleeve 17 fastened to the lower housing 12. Air passages52 in the body 47 allow air under pressure to escape around the hammerbody when the apparatus 10 is suspended above the borehole bottom 8(FIG. 1). As heretofore mentioned, a free flow of air prevents debris(and water) from contaminating the air percussion apparatus while themechanism is being tripped in and out of the borehole.

FIG. 6 details part of the clutch mechanism 56. This view locates thehelical groove engaging balls 58 at the bottom of the helix 36 in shank32 of piston 30 (FIG. 3). The balls 58 are retained in ball race 59; therace 59 being secured within ball and clutch housing 60.

FIG. 7 is a view taken through the clutch mechanism primarily comprisedof a multiplicity of "sprags" or clutch dogs 57 that allow rotation inone direction only. Since rotation preferably occurs only on the pistondownstroke, the clutch dogs 57 engage the balls within helical tracks 36resulting in a clockwise rotation of the piston and hammer bit asheretofore described. On the upstroke of the piston the clutch releasesthe ball driver mechanism. The piston then travels up the housing 12without rotation.

FIG. 8 illustrates the percussion tool 10 at the completion of anoperating cycle. The hammer has been rotated or indexed the preferred 4to 5 degrees prior to impact of the cutting face 19 of the hammer bitwith the formation bottom 8.

FIG. 9 is an alternative piston shank sliding engagement mechanism. Thepiston 130 forms an internal sleeve 142 with, for example, threeparallel, axially aligned semi-circular grooves 120 degrees apart formedin the sleeve wall of the body. The shank 150 of hammer bit 118 retainsthree ball bearings 160 that are aligned with each of the complimentarygrooves 143 formed in the piston body 131. The shank of the hammer bitthen is slideably "splined" to the piston with a minimum of drag.

The cross-section of FIG. 10 depicts the ball bearings 160 secured tothe shank 150 of the hammer bit 118 and slideably engaged with theparallel grooves 143 formed in the piston body 131.

It will of course be realized that various modifications can be made inthe design and operation of the present invention without departing fromthe spirit thereof. Thus, while the principal preferred construction andmode of operation of the invention have been explained in what is nowconsidered to represent its best embodiments, which have beenillustrated and described, it should be understood that within the scopeof the appended claims, the invention may be practiced otherwise than asspecifically illustrated and described.

What is claimed is:
 1. An air percussion hammer apparatus with means forrotating a hammer rock drill bit while it reciprocates in a housingindependent of an attached drill string, said hammer rock drill bitrotating means comprising:a cylindrical housing forming a first openup-stream end connectable to a drill string component and a seconddownstream end, said second end containing said hammer rock drill bit, apneumatic feed tube forming a first open end and second substantiallyclosed end, said first end of the feed tube is concentric with and fixedwithin said housing and positioned toward said first up-stream end ofsaid housing, the second end of the feed tube forms one or more meteredopenings between said first and second ends of the feed tube, a pistonbody is slidably retained within a first cylindrical sleeve formed bysaid housing, the piston body forms first and second open ends, saidfirst end is reduced in diameter, said first end further concentricallyretains and is slidably engaged with said second end of said feed tube,said second downstream end of said piston body engaged with said housingsleeve forms a hammer striking surface, said piston body further formsat least one axially oriented helical groove in an outside wall formedin said reduced diameter first upstream end of said piston body and atleast a pair of pneumatic communication ports between an outside wall ofsaid piston body and an interior chamber formed by said piston body, oneof said ports leads from an interior chamber formed by said piston bodytowards said second open end of said piston body to a chamber formedbetween the first open end of the piston body and said cylindricalhousing, the other of said ports leads from an interior chamber formedbetween an exterior wall of said piston body and said cylindrical sleeveformed by said housing toward said second open end of said piston body,one or the other of said ports in said piston body sequentiallyregisters with said metered openings in said feed tube when saidreciprocating piston body is moved into alignment therewith during anoperating cycle of said apparatus, said second end of the piston bodyfurther forms a longitudinally extended sleeve concentrically therein, ahammer rock drill bit body slidably contained within a second sleeveformed by said second end of said cylindrical housing, said hammer rockdrill bit body further forming an upstream shaft end adapted to beaxially slidable within said sleeve formed in said piston body, saidshaft end of said hammer rock drill bit body is rotationally keyed tosaid piston body by an engagement means such that the hammer rock drillbit body moves in concert with said piston body, said hammer drill bitbody forming a second downstream cutter end, and clutch means containedwithin said housing, said clutch means being positioned adjacent to andinterconnected with said at least one helical groove formed in saidfirst reduced diameter end of said piston body, the clutch means servesto rotate the piston body and the hammer rock drill bit body engagedtherewith incrementally and in one direction only, each time the pistonbody oscillates within the cylindrical housing during operation of theair percussion apparatus.
 2. The invention as set forth in claim 1wherein said engagement means that slidably keys the upstream shaft endof said hammer rock drill bit body to said sleeve formed by said pistonbody is at least a pair of longitudinally extending, oppositely opposedflats formed by said upstream shaft end, said piston body formingcomplimentary flats to slidably accept said upstream shaft end withinsaid sleeve.
 3. The invention as set forth in claim 1 wherein saidengagement means is at least a pair of spherically shaped detentsslidably engaged with complimentary shaped, longitudinally extendinggrooves formed in adjacent surfaces of said upstream shaft end of saidhammer rock drill bit body and said sleeve formed in said piston body.4. The invention as set forth in claim 3 wherein there are threespherically shaped detents connected to and extending from said upstreamend of said shaft of said hammer rock drill bit body, said detents beingpositioned about 120 degrees apart, said detents being slidably engagedwith complimentary shaped longitudinally extending grooves formed bysaid piston body in a wall of said sleeve.
 5. The invention as set forthin claim 1 wherein said engagement means is a multiplicity oflongitudinally extending splines formed by said shaft of said hammerrock drill bit body, said splines being slidably interfitted withsplines formed by said piston body in a wall of said sleeve.
 6. Theinvention as set forth in claim 1 wherein said piston body and hammerrock drill bit body is rotated by said clutch means on a downstroke ofthe piston body.
 7. The invention as set forth in claim 1 wherein therotational speed of the piston body and hammer rock drill bit body isbetween 10 and 20 RPM.
 8. The invention as set forth in claim 7 whereinthe rotational speed is about 20 RPM.
 9. The invention as set forth inclaim 1 wherein the hammer rock drill bit body impacts an earthenformation during operation of the air percussion apparatus about 1600beats per minute.
 10. The invention as set forth in claim 9 wherein therotational speed of the piston body and hammer rock drill bit bodyresults in an angular displacement of the hammer rock drill bit body ofabout 5 degrees per each impact of the bit against the formation. 11.The invention as set forth in claim 1 wherein the clutch means is asprag clutch.
 12. The invention as set forth in claim 1 wherein thesprag clutch allows the piston body and hammer rock drill bit body torotate on the downstroke only, the clutch releases on the upward cycleof the piston body.
 13. The invention as set forth in claim 11 whereinthe clutch is engaged with the helical groove formed in said reduceddiameter first upstream end of the piston body by a ball bearing, saidhelical groove being spherically shaped to match the diameter of theball bearing.
 14. The invention as set forth in claim 12 wherein thereare three helical grooves formed in said first upstream end of thepiston body 120 degrees apart, each of said grooves is engaged with aball bearing retained within said clutch means.
 15. A method of rotatinga hammer rock drill bit of an air percussion hammer bit apparatus whileit reciprocates in a housing, the rotating of the bit being independentof an attached drill string comprising the steps of;forming acylindrical housing with a first open upstream end connectable to adrill string component and a second downstream end, said second endcontaining said hammer bit, forming a pneumatic feed tube with a firstopen end and second substantially closed end, said first end of the feedtube is concentric with and fixed within said housing and positionedtoward said first upstream end of said housing, the second end of thefeed tube forms one or more metered openings between said first andsecond ends of the feed tube, forming a piston body that is slidablyretained within a first cylindrical sleeve formed by said housing, thepiston body forms first and second open ends, said first end is reducedin diameter, said first end further concentrically retains and isslidably engaged with said second end of said feed tube, said seconddownstream end of said piston body engaged with said housing forms ahammer striking surface, said piston body further forms at least oneaxially oriented helical groove in an outside wall formed in saidreduced diameter first up-stream end of said piston body and at least apair of pneumatic communication ports between an outside wall of saidpiston body and an interior chamber formed by said piston body, one ofsaid ports leads from an interior chamber formed by said piston bodytowards said second open end of said piston body to a chamber formedbetween the first open end of the piston body and said cylindricalhousing, the other of said ports leads from an interior chamber formedbetween an exterior wall of said piston body and said first cylindricalsleeve formed by said housing toward said second open end of said pistonbody, one or the other of said ports in said piston body sequentiallyregisters with said metered openings in said feed tube when saidreciprocating piston body is moved into alignment therewith during anoperating cycle of said apparatus, said second end of the piston bodyfurther forms a longitudinally extended sleeve concentrically therein,forming a hammer rock drill bit body that is slidably contained within asecond sleeve formed by said second end of said cylindrical housing,said hammer rock drill bit body further forming an upstream shaft endadapted to be axially slidable within said sleeve formed in said pistonbody, engaging means to slidably engage said shaft end of said hammerrock drill bit body to said piston body such that the hammer rock drillbit body moves in concert with said piston body, said hammer rock bitbody forming a second downstream cutter end, and interconnecting aclutch means to said piston body, said clutch means contained withinsaid housing, said clutch means being positioned adjacent to andinterconnected with said at least one helical groove formed in saidfirst reduced diameter end of said piston body, the clutch means servesto rotate the piston body and the hammer rock drill bit body engagedtherewith incrementally and in one direction only, each time the pistonbody oscillates within the cylindrical housing during operation of theair percussion apparatus.
 16. The method as set forth in claim 15further comprising the step of rotating the piston body and hammer rockdrill bit body on each downstroke of the piston. .[.17. An airpercussion hammer comprising:a cylindrical housing having an upstreamend having means for connecting to a drill string and a downstream endincluding means for mounting a hammer bit; a piston slidably retainedwithin the housing, a downstream end of the piston including a surfacefor striking a hammer bit mounted on the end of the housing; fluidporting in the housing for alternately driving the piston upwardly inthe housing and driving the piston downwardly in the housing forstriking a hammer bit; means for rotating the piston during the downwardstroke of the piston; means for preventing rotation of the piston duringthe upward stroke of the piston; and keying means for permittingrelative longitudinal movement while preventing relative rotationbetween the hammer bit and the piston..].18. .[.The invention as setforth in claim 17.]. .Iadd.An air percussion hammer, comprising:acylindrical housing having an upstream end having means for connectingto a drill string and a downstream end including means for mounting ahammer bit .Iaddend..[.wherein the hammer bit is mounted.]. forreciprocal movement in the housing.Iadd.; a piston slidably retainedwithin the housing, a downstream end of the piston including a surfacefor striking a hammer bit mounted on the end of the housing; fluidporting in the housing for alternately driving the piston upwardly inthe housing and driving the piston downwardly in the housing forstriking a hammer bit; .Iaddend..[.and characterized by.]. means forventing fluid from the hammer when the hammer bit is relatively down inthe housing and for applying fluid pressure for driving the piston whenthe hammer bit is relatively up in the housing.Iadd.; means for rotatingthe piston during the downward stroke of the piston; means forpreventing rotation of the piston during the upward stroke of thepiston; and keying means for permitting relatively longitudinal movementwhile preventing relative rotation between the hammer bit and thepiston..Iaddend..[.19. The invention as set forth in claim 17 whereinthe hammer bit is slidably contained within the downstream end of thecylindrical housing, the hammer bit comprising an upstream shaft endaxially slidable within the piston, the shaft end of the hammer beingrotationally keyed to the piston by an engagement means such that thehammer bit rotates in concert with the piston..]..[.20. The invention asset forth in claim 19 wherein the clutch means comprises at least a pairof spherically shaped detents for slidably engaging complementary shapedhelical grooves in the upstream end of the piston..]..[.21. Theinvention as set forth in claim 17 wherein the keying means forpermitting relative longitudinal movement while preventing relativerotation between the hammer bit and the piston comprises threespherically shaped detents between the upstream end of the shank of thehammer and the inside of the piston, the detents being positioned about120 degrees apart and slidably engaged with complementary shapedlongitudinally extending grooves..]..[.22. The invention as set forth inclaim 17 wherein the means for rotating the piston during the downwardstroke of the piston comprises a helical groove in the piston with ahelix angle and length sufficient for rotating the piston approximatelyfive degrees per cycle of the piston..]..[.23. The invention as setforth in claim 17 wherein the means for rotating the piston during thedownward stroke of the piston comprises a helical groove in the pistonand a ball bearing engaging the housing and the helical groove..]..[.24.The invention as set forth in claim 23 wherein there are three helicalgrooves formed in the upstream end of the piston 120 degrees apart, eachof the grooves being engaged with a ball bearing retained within thehousing..].25. A method of rotating a hammer rock bit of an airpercussion hammer bit apparatus while it reciprocates in a housing, therotation of the bit being independent of an attached drill stringcomprising the steps of:forming a cylindrical housing having an openupstream end connectable to a drill string component and a downstreamend containing a hammer bit, the hammer bit being free to reciprocatelongitudinally in the housing; connecting the upstream end of thecylindrical housing to the drill string; mounting a longitudinallymoveable annular piston on the housing defining a first chamber havingan upstream end and a downstream end having a hammer striking surface,and including a helical groove in an outside wall of the piston;alternately passing air from a chamber above the upstream end of thepiston to the inside of the piston, and passing air from the inside ofthe piston to a chamber outside of the piston adjacent to the downstreamend of the piston, for reciprocating the piston in the housing andstriking the hammer bit; engaging the helical groove with a detent forrotating the piston during a downward stroke of the piston; rotating thehammer bit in concert with the piston; and preventing rotation of thepiston during an upward stroke of the piston.